Fisher was in full cardiac arrest on the plane and rushed to a local hospital. She was reported to be in stable condition at Ronald Reagan UCLA Medical Center several hours later.

Her brother, Todd Fisher, told The Associated Press that she was "out of emergency" and stabilized. He said he could not discuss any other details about what happened.

Passengers were administering CPR on the flight heading from London to Los Angeles International Airport, according to TMZ.

A source told NBC that her condition was "not good."

RAW: Carrie Fisher Talks in Berkeley About Dying, Humor

Actress Carrie Fisher talks about Wishful Drinking, the one-woman show she created and performs that's soon headed to Broadway. She spoke to NBC Bay Area in 2009.

(Published Tuesday, Dec. 27, 2016)

United Airlines released a statement about reports of an "unresponsive" passenger on one of their flights, but did not reference Fisher in its comments.

"Medical personnel met United flight 935 from London to Los Angeles upon arrival today after the crew reported that a passenger was unresponsive. Our thoughts are with our customer at this time, and any requests for additional information should be directed to local authorities," the statement read.

The Los Angeles Fire Department said paramedics "were standing by for the plane's arrival, provided Advanced Life Support and aggressively treated and transported the patient to a local hospital."

A passenger on board the flight told NBC4 the crew made an announcement 20 minutes before the flight landed to ask if there were any nurses or doctors on board.

Fisher is best known for her role as Princess Leia from "Star Wars."

Mark Hamill, Fisher's co-star in the "Star Wars" movies, tweeted that he is "sending all our love" to Fisher.

as if 2016 couldn't get any worse... sending all our love to @carrieffisher